A battery is a device that stores electric charge for use as a power source. The charging process is based on a chemical reaction that takes place between an electrolyte and two electrodes called an anode and cathode. The capacity to store electric charge is a function of the surface area of these electrodes and the particular electrolyte used.
Common types of batteries include sealed lead acid (SLA) batteries, nickel-cadmium (Ni—Cd) batteries, and lithium-ion (Li-Ion) batteries. SLA batteries can hold a charge for up to three years and are generally used to provide backup power during emergencies. Ni—Cd batteries provide a fast, even energy discharge and are most often used to power appliances and audio and video equipment. Li-Ion batteries have the highest energy storage capacity (generally twice the capacity of Ni—Cd batteries) and are used to power portable computers, cellular phones, and digital cameras to name a few.
Another type of battery known as a double-layer capacitor stores energy based on a microscopic charge separation that takes place at an electrical-chemical interface between an electrode and electrolyte. The capacitor is charged by a primary energy source and then discharged when connected to a device to be powered, generally referred to as a load. The charging and discharging process is repeatable; that is, after discharging takes place through the load the capacitor may be recharged by connecting its electrodes to the primary energy source. Double-layer capacitors have been used to power bulk electronic devices including radios, motors, and the like.
On a vastly smaller scale and more particularly in the field of large scale integration, solid-state capacitors are used to store charge. Unlike double-layer capacitors and other types of electrochemical batteries, solid-state capacitors store energy in the form of an electrostatic field between a pair of conductive layers separated by a dielectric material. In these devices, capacitance is directly proportional to the surface areas of the conductive layers and is inversely proportional to the separation distance between these layers. Capacitance also depends on the dielectric constant of the material separating the layers.
Because solid-state capacitors are only formed from two conductive layers, they are limited in terms of the amount of voltage they can store. Attempts have been made to increase the storage capacity of solid-state capacitors by increasing the surface area of the conductive layers. This approach, however, has proven to be undesirable because the increased surface area consumes an excessive amount of die space. A need therefore exists for an improved energy storage device for use in integrated circuits.